Worldwide, some 6,000 “songbird” species are put by taxonomists into a single order (Passeriformes, but don’t worry, that won’t be on the quiz). The members of this order all share a common ancestor somewhere in the distant past, and they are all more or less highly adapted to singing, having a complex organ for sound production, called a syrinx, and brains specialized for making and interpreting sounds. In the same way that language is a central characteristic of the human species, and one that has profoundly shaped our evolution and our culture, song is the central biological fact of a songbird’s life.
It is certainly the central fact for our local mockingbird, who has been singing about 20 hours a day lately, including vigorous, non-stop sessions between about midnight and 3 am. This level of activity may not be typical for songbirds generally, but it is for male mockingbirds at this season, especially unmated ones. Females, as is true of most songbird species, listen and call but don’t generally sing. While a mockingbird’s song is more complex than that of most species, as well as more persistent, it’s typical in other ways and furnishes a good example of several universal principles about bird song.
For one thing, though it may sound like his efforts are infinitely varied, a mockingbird in fact has a finite number of song elements at his disposal — though he learns new ones and discards old ones during the course of a season. But while a song sparrow may know four or five song types, and a red-eyed vireo might alternate among 12 or 15, a mockingbird’s repertoire may be in the hundreds. Some of these songs are simple sounds — buzzes, and whistles that the bird may produce by instinct. But most of this repertoire is either improvised or learned, and indeed the species gets its name from its ability to imitate the songs of other species. As you might suspect, this vocal flexibility is associated with a highly developed syrinx and brain: a mockingbird is truly born to sing.
For another thing, each mockingbird’s repertoire is unique, an idiosyncratic mix of improvisations and copied notes that strike the bird’s fancy. Other songbird species vary widely in how much variation the songs of individuals show, but for many species, a particular bird’s song may be readily recognizable to an attentive human listener, let alone to members of the bird’s own species. Birds can tell, in other words, who exactly among their neighbors is singing, and they know instantly if a new male shows up in the area. Songbirds live in a fabric of sound and constantly glean social information from the noise around them.
Since sleep is impossible when our mockingbird is giving his nocturnal concerts, I amuse myself by keeping track of the species he mimics and trying to track how many unique songs he sings before he begins repeating himself. As for the number of songs, I can’t really say: after a dozen or two song elements, I lose track of what he has already performed, highlighting one way in my brain works less well than his! Predictably, the notes of most of the other common nesting birds in my part of the Island figure in his song — blue jay, robin, flicker, great crested flycatcher, and so on. But our mocker also learned a commendable common tern call before I had seen (or heard) my first tern of the season, and I’m not sure where he learned the distinctive call of a scarlet tanager, a species that rarely turns up near where I live.
Research has shown that singing ability in part determines a male mocker’s attractiveness to the ladies: females will mate preferentially with the males that have the most varied song. There may be good reason behind this: the males with the broadest repertoire are likely to be the oldest and most experienced, and hence likely to be the best at providing food for young or an incubating female. And I don’t know whether it has been shown to be the case, but I would not be surprised to find that a female mocker can compare a male’s “play list” to her own knowledge of what is singing locally, and so be able to judge whether a prospective mate is a local bird, with local knowledge, or a transient, singing a good song but not knowing his way around town.
We’re in the season of birdsong now, the best time to begin learning the voices of your avian neighbors. Mockingbirds — widespread, unmistakable, and easy to study — may be the best species to begin paying attention to. But once you’ve grown familiar with their song, keep listening and try to learn other species, and then to recognize the individuals that live near you. Birds structure their social worlds with song; by listening in, you can learn hidden details of their lives.